1. Field of the Invention
This invention relates to a fixing device using a reinforced thin cylinder structure, a thin cylinder structure used for the fixing device, and a method for manufacturing thereof.
2. Description of the Related Art
Copy machines utilizing an electrophotographic process require a process for fixing the toner formed on a recording sheet to convert it to a permanent image, various fixing method such as the solvent fixing, pressure fixing, and heat fixing have been known as the fixing method.
The solvent fixing is disadvantageous in that solvent vapor is transpired to cause odor and health problems, and the pressure fixing is disadvantageous in that the flexibility is poor in comparison with other methods and pressure sensitive toner to be used in this method is expensive. Therefore, these two methods have been not used practically.
Accordingly, as a fixing method of a toner image for copy machines utilizing an electrophotographic process, the heat fixing in which toner is melted by heating to fix the image on a recording sheet has been used popularly.
FIG. 1 is a diagram for illustrating the schematic structure of a fixing device utilizing the heat fixing. As shown in FIG. 1, the fixing device comprises a heating roller 30 provided with a heater 31 in a cylindrical metal core 32 and a releasing layer 33 comprising a heat resistant resin coating layer or heat resistant rubber coating layer formed on the outside peripheral surface of the metal core 32, and a pressure roller 40 provided in contact with the heating roller with the pressure roller comprising a cylindrical metal core 41 and a heat resistant elastomer layer 42 formed on the outside peripheral surface of the metal core. The fixing processing is carried out by passing a sheet 43 on which a toner image 44 is formed between the heating roller 30 and pressure roller 40, then, additives contained in the toner are fused so that the image is fixed on the sheet 43.
The heating roller method in which a heating roller is used to fix as a fixing device utilizing heat fixing is advantageous in that the thermal efficiency is high in comparison with other heat fixing methods such as hot air fixing and oven fixing, therefore required electric power is low (energy saving) and copying speed is high, and also the risk of fire due to paper jamming is low, therefore the heating roller method has been used popularly.
Conventional heating roller type fixing devices require a long time for raising up the surface temperature of the heating roller from a room temperature to a prescribed setting temperature (referred to as warm-up time hereinafter), namely 1 minute to 10 minutes. The warm-up time is determined based on the relationship between the heat capacity of the heating roller and supplied power. Therefore if the heat capacity of the heating roller is small and supplied power is large, the warm-up time can be shortened. However, the reduction of the heat capacity of the roller is limited by the rigidity of the roller, and the increase of supplied power is also limited by the power consumption of the copy machine.
In general, power to be exclusively supplied to a fixing device ranges from 300 W to 1000 W. In order to shorten the warm-up time within this power range, it is most effective to reduce the heat capacity of the heating roller.
If the diameter of a roll and wall thickness of a roll core are reduced to decrease the roll heat capacity of a roller to be used for a fixing device, the roll rigidity is reduced with the reduction of the diameter or wall thickness. The roll heat capacity is proportional to the square of roll diameter, while the roll rigidity is proportional to the biquadrate of roll diameter. When a load which is necessary for the fixing roller to function sufficiently is applied on the fixing roller having a small diameter, the load causes deformation in the axial direction of the roller (referred to as deflection hereinafter) and deformation in the peripheral direction (referred to as collapse hereinafter). As described above, the reduction of the roll diameter beyond a certain value results suddenly in the reduction of roll rigidity, thus a load sufficient for fixing can not be loaded.
Particularly, collapsing of a roller out of the above-mentioned two types of deformation severely affects on the function of the fixing device, namely the function of the heat fixing process. In detail, the function for carrying papers by pinching and the function for uniform heat transferring to toner layer are not quite performed, thus the fixing device does not function as a fixing device. Therefore, the wall thickness should be more than a prescribed value which is sufficient for a prescribed roll rigidity.
In general, pressure of 0.5 to 5.0 kg/cm.sup.2 in nipping area of a pair of rollers comprising a heating roller and pressure roller is necessary for performing prescribed fixing function. Interstices due to the roughness on the surface of a paper and roller can not be eliminated by a pressure lower than the prescribed value, heat is not transferred efficiently to powder toner. Such heat transfer causes mottled fixed toner surface, namely poor image quality.
Since the total load between both rollers is as high as 20 to 200 kg for loading sufficient pressure necessary for fixing, the roll rigidity of a heating roller which is resistant to such heavy load is necessary for a roller used as a heating roller (cylinder), and the minimization of roll diameter and the thinning of roll wall are inherently restricted.
In view of such problems, various techniques for preventing collapse of thin rollers have been proposed. These techniques for preventing collapse of rollers and problems which occur when such rollers are used for a fixing device are described hereinunder.
As the first method, for example, Japanese Unexamined Utility Model Application No. 59-128665, Japanese Examined Utility Model Application No. 61-4926, Japanese Unexamined Patent Application No. 61-59381, and Japanese Unexamined Patent Application No. 6-130845 describe a method in which a reinforcing member is provided in a roller to prevent collapsing, thus the roll strength is increased without increasing in heat capacity of the roll itself. FIG. 2 is a cross-sectional view for illustrating the first example of a conventional fixing roller structure. In FIG. 2, the numeral 1 represents an inner frame of a supporting member of a roller, the numeral 2 represents a supporting rod for supporting the inner frame, the numeral 3 represents a roller (sleeve), the numeral 4 represents thermal insulator, the numeral 5 represents a coil heater, namely a heating source, the numeral 6 represents an offset preventing layer, namely a releasing layer, and the numeral 7 represents a supporting shaft.
For such a fixing roller having a structure as shown in FIG. 2, the supporting rod 2 for supporting the inner frame 1 of the reinforcing member for the roller 3 which is a constituent of a fixing roller is required, that is, large members for inner reinforcing structure are required. Such structure results inevitably in the large size and increased cost. In addition, the placing of large members in a fixing roller results in increased heat capacity of the whole fixing roller, so such structure is not preferable from the view point of the reduction of warm-up time. The portion of contact between the inner frame 1 and roller 3 should be structured using thermal insulator 4 such as resin. If a material having a thermal expansion coefficient smaller than that of metals is used, the inner frame 1 can lose contact with the inner surface of roller 3 when the fixing roller is heated, which is an disadvantage.
FIG. 3 is a cross-sectional view for illustrating the second example of a conventional fixing roller structure. As shown in FIG. 3 depicting the second method, a roller 8 is provided with thick portions 9 each having the wall being formed partially thick in the axial direction. As a modified example of this case, a structure in which ribs for reinforcement are provided instead of thick portion 9 each having the wall being formed partially thick in the axial direction is proposed. Examples of the second method is described in, for example, Japanese Unexamined Utility Model Application No. 56-7949 and Japanese Unexamined Patent Application No. 57-155571.
For manufacturing a fixing roller having a structure according to the second method, the wall should be so processed that the wall thickness of a roller 8 varies partially in the axial direction to form thick portions 9 inside the cylindrical structure. Therefore the heat capacity varies along the axial direction partially at the thick portions. if this type of heating roller is used for heat fixing, and the roller is heated, the thick portions cause ununiform temperature distribution, thus the uniform fixing inadvantageously failed. Further, in manufacturing of rollers, the drawing method can not be used for manufacturing seamless pipes, the manufacturing process of such roller results in higher cost.
FIG. 4 is a cross-sectional view for illustrating the third example of a conventional fixing roller structure.
As shown in FIG. 4 depicting the third method, a solid pipe is inserted into the internal of a cylinder to improve the rigidity of a thin cylinder as described in Japanese Unexamined Patent Application No. 63-26752. In the case of the roller shown in FIG. 4, reinforcing rigid members 11 are inserted to a conductive cylinder member 10, which is served as a photosensitive member, from both ends to improve the rigidity of the conductive cylindrical member 10, In this case, the contact charging conductive rubber roller 12 functions as a pressure roller.
In the third method, the roller essentially has high rigidity. However, because the reinforcing rigid member 11 is a solid member, a heat generating mechanism can not be provided when the roller is used as a fixing roller. This type of fixing roller can not be used for heating roller type fixing devices. Further, even when such roller is used for fixing devices of surface heat generating type (roller pair type) having no heat generating mechanism in the internal of the roll, such type of roller is not preferable from the view point of low power performance (energy saving) because of the surface temperature rising loss due to transfer of heat to the internal. The weight due to the reinforcing rigid member 11 is added, the roller has the same roll rigidity as that of a roller designed to have a thick wall, and such requirement for the extra member results in cost up. The reinforcing rigid member 11 is a member having large heat capacity, it means that a member having a large heat capacity is provided in the internal of the roller, such high heat capacity structure is not preferable also from the view point of shortening of warm-up time.
FIG. 5 is a cross-sectional view for illustrating the fourth example of a conventional fixing roller structure. In a thin cylindrical structure according to the fourth method, an inner core formed of thin plate is inscribed inside a thin cylindrical outer cylinder and fixed as described in Japanese Unexamined Patent Application No. 1-126417. In detail, as shown in the cross-sectional view of FIG. 5, the roller has a structure that the inner core 14 in the form of a triangular prism formed of thin plate is inscribed inside the cylindrical outer cylinder 13 and fixed. In the case of the structure in accordance with the fourth method, it is difficult to provide a infrared ray lamp heater in the internal as a heating source. From the viewpoint of the strengthening of a roller, the rigidity in the direction of the axis of the roller is improved, but collapsing due to the thinning of the wall in the peripheral direction is not prevented at the area where the inner core 14 is inscribed, therefore such structure is not preferable from the view point of reliability.
The present invention is accomplished in view of the above-mentioned various problems, it is an object of the present invention to provide a small-sized high performance fixing device using a thin cylindrical structure, which thin cylindrical structure receives the external force at the peripheral surface, having a structure which is free from the problem of loss of contact between the inner reinforcing member and the thin cylindrical structure and prevents sufficiently the thin cylinder from being collapsed due to the thinned wall in the peripheral direction. Further, it is another object of the present invention to provides a thin cylindrical structure used for fixing devices and a method for manufacturing the thin cylindrical structure.